1. Field of the Invention
The present invention relates to receiving apparatus including adaptive beamformers. In particular, the present invention can provide receiving apparatus for use in a base station of a cellular mobile communication system.
2. Description of the Related Art
In a cellular mobile communications system, one of the main tasks of the base station is to detect the signal of each wanted user (i.e. each active mobile station) in a multi-user and multi-path environment. In order to achieve satisfactory signal detection at low bit error rates, two conditions must be satisfied. Firstly, the power level of the signal received by the base station from the mobile station must be greater than a certain threshold value. Secondly, the multi-user interference (MUI), sometimes referred to also as multiple access interference (MAI), must be reduced to an acceptable level.
To satisfy the two conditions identified above, it is effective to use adaptive beamformers in general and digital beamformers in particular. The principle underlying a digital beamformer is to form a spatial beam pattern in such a way that the angles of arrival of wanted signals fall well within a main lobe of the beam pattern whereas the interfering signals are located as much as possible in the nulls, low side lobes or boundary regions of the main lobe.
FIG. 1 of the accompanying drawings shows parts of a previously-considered digital beamformer. A plurality of independent sensors (antenna elements 21 to 24) are provided to detect, at different points in space, a transmission signal sent to the base station by a mobile unit (not shown in FIG. 1). The antenna elements 21 to 24 permit sampling of the received signal in space. The respective receive signals produced by the antenna elements 21 to 24 are digitised (e.g. by RF down-converters not shown) and then applied to the digital beamformer 6 which is employed as a spatial filter.
The digital beamformer 6 includes a set of complex number multipliers 8 connected respectively for receiving the different antenna signals. Each complex number multiplier multiplies its antenna signal by a weight value W set by a weight setting unit 12 of the beamformer 6. The resulting outputs of the multipliers 8 are then combined by a combiner 10 to produce an output signal of the digital beamformer. The object of the spatial filtering carried out by the digital beamformer 6 is to optimise the beam former response with respect to some prescribed criterion so that noise and interference are minimised in the output signal.
A subtractor 11 subtracts a reference signal from the output signal of the digital beamformer to produce an error signal. The weight setting unit 12 receives the error signal and the receive signals from the antenna elements 21 to 24 and processes them to derive the weight values W1 to W4 applied to the complex number multipliers 8.
In a steady-state condition, in which the wanted and interfering signals each have a fixed angle of arrival at the receiving apparatus, there will be a fixed optimum set of beamformer weight values W1 to W4 which satisfies the prescribed criterion for minimising noise and interference at the output of the beam former. An adaptation algorithm is employed in the weight setting unit 12 which, in the above steady-state condition, would cause the weight values to converge to their optimum steady-state values and, thereafter, the noise and interference at the output of the beamformer would remain at a minimum level related to the number of weights. However, in practice, multi-path propagation means that the transmission channel between the subject mobile unit and the base station is time-variant and, furthermore, the positions of the interfering signal sources (for example other mobile stations) will change, with respect to one another and the base station, over time. Accordingly, using its adaptation algorithm, the weight setting unit 12 is required to update the beamformer weight values continuously in accordance with the changing operating parameters.
Incidentally, further information about digital beamforming techniques and related adaptive algorithms can be found, for example, in “Digital beamforming in wireless communications”, John Litra and Titus Kwok-Yeung Lo, Artech House Publishers, 1996, ISBN: 0-89006-712-0, the content of which is incorporated herein by reference.
In practice, when a base station is expecting to receive a signal from a wanted user, it initially has no idea of the direction from which that signal will come. Thus, it is inappropriate to point the initial beam pattern, which is determined by the initial weight setting of the digital beamformer, to any particular direction. However, if an omnidirectional initial beam pattern is used, the level of the MUI can be so high that it takes a long time for the adaptation algorithm to converge, which inevitably leads to long delay and/or waste of bandwidth.
A paper entitled “An adaptive array antenna using combined DFT and LMS Algorithm” by K Watanabe, I Yoshii, and R Johno, Annual EIT Conference of Telecommunications, 1997, discloses a two-stage approach for determining beam directions in a TDMA communications system. In the first stage, in place of a digital beamformer (weighted summing circuit) discrete fourier transform (DFT) processing is applied to the received signals so as to effectively form plural fixed beams. The results of the DFT processing are used to establish the initial weight factors for the second stage which involves Least Mean Square (LMS) processing of the received signals. This two-stage approach is partially effective in improving the convergence of the beamformer weights. However, it suffers from the following serious limitations. Firstly, it is necessary for all the beams formed during DFT processing to be produced simultaneously, making the hardware construction of the receiving apparatus expensive. Secondly, the number of fixed beams is limited to being no greater than the number of antenna elements. Thirdly, there is no control over the beam shapes and pointing directions during the DFT processing. In particular, there is no freedom in choosing the “look-directions” of the antenna elements and the parameter d/λ independently (d is the inter-element spacing and λ is the operating wavelength). This lack of control effectively limits the DFT approach to use in switched beam and multi-beam antennas only when the beams are pointed to certain directions and no sidelobe control is needed. In practice the antenna elements must therefore be evenly spaced and placed on a plane. Fourthly, the DFT approach is not fully effective in the case in which two or more of the fixed beams provide comparably-good signals.
Accordingly, it is desirable to provide a technique which enables the base station beamformer to set up its initial weights quickly, thus reducing the convergence time and the demand for long pilot signals, without the limitations mentioned above.